Unfortunately we have our first problem that could really be counted as security hole.
Giving "1 LIST .. *" IMAP command allows the user to see all files and directories under the mbox root's parent directory, so potentially you could see other users' mailbox names. Nothing can be done with them though, so it's not possible to read or modify them.
There are also some other less than wanted behaviors that can be done with "." and ".." directories, such as "DELETE .." command can give errors. Also with maildir if you store indexes in different directory than mails this command can be used to delete the index files. Anyway I don't see any of these as security issues.
Hopefully most people have their mail directory under user's home directory, so this would only list the home directory's contents.
But if you do have users' mails stored in eg. under /var/mail/%u/ directory, giving that command would list everything under /var/mail, assuming that the process has permissions to do that (that's why you should be using different UIDs for users if possible).
This affects 1.0 beta and 1.0 stable releases, but not 0.99.x.
Fix is in http://dovecot.org/list/dovecot-cvs/2006-May/005563.html
I'll also release 1.0 beta8 now.
Thanks for Bill Boebel for noticing "DELETE .." doing weird things with maildir.